1. Field of the Invention
The present invention relates to a keyboard, which is used as and suited to an input device for word processors, personal computers and the like.
2. Description of the Related Art
A conventional keyboard used in such an input device has membrane switches that are placed on a support plate (not shown) made of metal. As illustrated in FIGS. 9 and 10, each of the membrane switches comprises a lower sheet 50, which has a stationary contact (not shown), and a flexible upper sheet 52 that has a movable contact (not shown) and that is arranged on this lower sheet 50 through an insulating spacer 51.
A support member 53 made of a resin material is arranged on each of the membrane switches. As viewed in each of these figures, two engaging portions 53a, each having an engaging hole 53b, are formed on the left-side portion of this support member 53, while two engaging portions 53c, each having an engaging hole 53d, are formed on the right-side portion thereof.
An opening 53e is formed among the left-side engaging portions 53a and the right-side engaging portions 53c.
Further, a first lever 54 made of a resin material has a pair of arm portions 54c and 54d each having cylindrical convex portions 54a and 54b respectively provided at both ends thereof. Moreover, a pivotally supporting portion 54e projecting outwardly is formed at the central portion of each of the arm portions 54c and 54d. Furthermore, each of the convex portions 54b is rotatably engaged with a corresponding one of the engaging portions 53d of the support member 53.
A second lever 55 made of a resin material is disposed outside the first lever 54. As viewed in FIG. 10, this second lever 55 comprises a connecting portion 55a provided at the right-side portion thereof, and a pair of arm portions 55b, which extend leftwardly upwardly and downwardly from the top and bottom portion of this connecting portion 55a, respectively. The second lever 55 is nearly U-shaped.
Cylindrical convex portions 55c are formed at both end portions of the connecting portion 55a, respectively, in such a manner as to protrude outwardly therefrom. A cylindrical convex portion 55d is formed at the left-side end portion of each of the pair of arm portions 55b in such a way as to project outwardly therefrom. A nearly elliptic recess portion 55e is formed at the central portion of each of the pair of arm portions 55b. Furthermore, each of the convex portions 55d is rotatably engaged with a corresponding one of the engaging portions 53a of the support member 53. Additionally, each of the pivotally supporting portions 54e of the first lever 54 is fitted into the corresponding recess portion 55e, so that the first and second levers 54 and 55 are combined with each other in such a manner as to intersect each other.
Further, a pair of holding portions 56b each having a sliding groove formed in such a way as to be outwardly opened are provided on the left-side portion of the back surface of a key top 56 made of a resin material, as viewed in these figures, in such a way as to face each other. Moreover, a pair of holding portions 56d each having a sliding groove formed in such a way as to be inwardly opened are provided on the right-side portion of the back surface of the key top 56, as viewed in these Figures, in such a way as to face each other.
As viewed in these figures, each of the protrusions 54a of the first lever 54 is slidably held in a corresponding one of the left-side holding portions 56b, while each of the protrusions 55c of the first lever 55 is slidably held in a corresponding one of the right-side holding portions 56d. Thus, the key top 56 is mounted on the first and second levers 54 and 55 in such a manner as to freely move up and down.
Furthermore, a dome-like elastic element 57 made of insulating rubber is put on the membrane switch by being placed in the opening 64e of the support member 53. The top end portion of the elastic element 57 abuts against the back surface of the key top 56.
The conventional keyboard is constituted by placing a plurality of sets of the first and second levers 54 and 55, the key top 56 mounted on these levers 54 and 55, and the elastic element 57 on the membrane switch in a plurality of lines.
Further, when the key top 56 is pushed, first, the convex portions 54b and 55d of the first and second levers 54 and 55 respectively rotate around the engaging portions 53c and 53a , each of which is used as a fulcrum, of the support member 53. Moreover, the pivotally supporting portion 54e of each of the first levers 54 moves in a corresponding one of the nearly elliptic recess portions 55e. Furthermore, while the convex portions 54a and 55c slide in the sliding grooves 56b and 56d provided at the side of the key top 56, respectively, the first and second levers 54 and 55 move down, so that the key top 56 descends.
This descent of the key top 56 results in the elastic element 57 being pushed. When the dome-like elastic element 57 is dented or reversed, a switch substrate 52 is pressed by a projection 57a provided in the elastic element 57. Thus, a circuit contact provided on the switch substrate 52 touches an electric conductor provided on an insulating substrate 50. Consequently, the membrane switch is turned on.
Subsequently, when the pressing of the key top 56 is stopped, the resilient force of the elastic element 57 causes the first and second levers 54 and 55 and the key top 56 to return to the positions taken by these elements before the key top 56 is pressed. Moreover, the switch substrate 52 pushed by the elastic element 57 returns to an initial state thereof by a restoring force thereof. Thus, the engagement between the switch substrate 52 and the insulating substrate 50 is canceled. Consequently, the membrane switch is brought into an off-state.
However, in the case of the conventional keyboard, when the key top 56 descends, the connecting portion 55a of the second lever 55 is placed on the right-side arm portion 54d of the first lever 55, as viewed in these Figures. Thus, the first and second-levers 54 and 55 overlap each other. Consequently, each of the first and second levers 54 and 55 cannot be put into a completely horizontal position. Therefore, the conventional keyboard has a problem in the reduction in thickness thereof.
Further, ascending and descending operations of the key top 56 are unstable because of the facts that the first and second levers 54 and 55 are rotatably engaged with the engaging portions 53a and 53c , respectively, and that the pivotally supporting portions 54e each serving as a fulcrum of the first and second levers 54 and 55 moves by sliding in the recess portion 55e. Thus, the conventional keyboard has another problem in the operability of the key top 56.